:: Officials at the UK Foreign Office say they are "urgently" investigating whether any British nationals were involved.

:: A BP spokesman confirms a "security incident" at the facility, activates its emergency response system and sets up a helpline for relatives.

1.45pm

:: Downing Street officials confirm Prime Minister David Cameron will chair a meeting of the Government's crisis committee Cobra later in the day.

2.30pm

:: The Foreign Office confirms that British nationals are caught up in the attack.

3pm

:: Officials say no family hotline has been set up for the incident because the number of British nationals involved is "very small".

4.20pm

:: Reports from Algeria's official news agency APS, attributed to provincial authorities, say one British and one French national were killed in the attack.

:: After a 45-minute meeting of Whitehall's Cobra emergency committee, Mr Cameron's official spokesman says he cannot confirm that a Briton has died but says "several" UK nationals are involved.

He says Mr Cameron will speak to his Algerian counterpart Abdelmalek Sellal on Thursday.

Downing Street says it is "too early to speculate" on the motive for the attack, or whether it is linked to French support - backed by the UK - for Mali's government in its fight against Islamist forces which have occupied the north of the country.

:: It is reported that a news agency in the Saharan state of Mauritania was contacted by the militant group Katibat Moulathamine - "The Masked Ones" - with a claim that yesterday's attack was carried out by one of its affiliates, identified as "Those who sign their names in blood".

A spokesman for Katibat tells the Sahara Media Agency that 41 Westerners of nine or 10 nationalities were taken hostage, including seven Americans.

Five foreigners were being held in a factory, while 36 others were in living quarters at the plant, claims the spokesman, who says the action was carried out in retaliation for Algeria allowing France to use its airspace to carry out raids on northern Mali. Algerian workers are said to have been released.

7pm

:: Algerian Interior Minister Daho Ould Kabila tells reporters the militants wanted to leave the country with the hostages, but he had refused to let them go, the BBC reports.

He says: "It seems [the militants] want to leave the country with the hostages - this is completely unacceptable for the Algerian authorities.

"We reject all negotiations with the group, which is holding some 20 hostages from several nationalities."

Mr Kabila says the kidnappers are Algerian and operating under the order of Mokhtar Belmokhtar, a senior commander of al Qaida in the Islamic Maghreb (AQIM) until late last year, when he set up his own armed group, the BBC says.

Mr Kabila says as well as a Briton and Algerian national being killed, two other Britons, a Norwegian, two police officers and a security guard were also hurt in the gun battle.

"Since then, they've been facing off. The security forces are consolidating their position around the base," he says. Nearby border crossings have been closed as a precaution, he adds.

:: A man claiming to be a spokesman for the militants tells BBC Arabic that al Qaida carried out the attack.

He says they are only holding foreign nationals at the gas facility and had allowed Algerian workers to leave. The spokesman tells the BBC a list of demands has been sent to Algerian authorities, and the hostages will be killed if troops attempt to rescue them.

''Storming the gas complex would be easy for the Algerian military, but the outcome of such an operation would be disastrous," he warns.

:: US Secretary of Defence Leon Panetta condemns the attack and says he could not confirm the exact number of Americans seized, the BBC reports.

He says: "By all indications this is a terrorist act and the United States strongly condemns these kinds these kinds of terrorist acts."

Thursday January 17, 3.30am

:: Speaking from Australia, where he is attending the Australia-UK Ministerial Consultations, Foreign Secretary William Hague says there is no excuse for the attack, which he calls "cold-blooded murder".

He says: "This is an absolute tragedy, of course, in this dangerous and rapidly developing situation. The next of kin have been informed.

"The Government's Cobra emergency system is in full operation, I have spoken to our ambassador in Algeria and dispatched a rapid deployment team to Algeria to strengthen our embassy there and help them in their work.

"Excuses being used by terrorists and murderers who are involved, there is no excuse for such behaviour, whatever excuse they may claim.

"It is absolutely unacceptable, of course. It is in this case the cold-blooded murder of people going about their business. So there is no excuse, whether it be connected to Libya, Mali or anywhere else."